The invention is based on a metal halide lamp with a ceramic discharge vessel according to the preamble of claim 1. It relates in particular to metal halide lamps with an output of at least 100 W.
EP-A-587,238 has already disclosed a metal halide lamp with a ceramic discharge vessel and halide-resistant lead-through of the generic type. The front part of the lead-through, facing toward the discharge, may comprise an electrically conductive cermet (with a ceramic phase and a conductive phase). The ceramic phase is aluminum oxide or MgO, Sc2O3 or Y2O3. The conductive phase which is proposed for the cermet is a halogen-resistant metal, for example tungsten, or molybdenum disilicide (MoSi2). In these lamps, it is customary to use fill constituents comprising halides of the rare-earth metals (RE). DyI3 is recommended in this document. As an alternative, it is recommended to use the iodides of Sc, Y, Ho or Tm.
EP-A-887,839 recommends that a continuous cermet pin be used as a lead-through for metal halide lamps with a ceramic discharge vessel.
The drawback of these designs is that a large proportion of the ions of the rare-earth metals which are formed in the fill become bonded through reacting with the ceramic, usually aluminum oxide, even after a short operating period. Therefore, it has hitherto been necessary to use a considerable excess, but this is somewhat undesirable in view of the corrosive properties. Alternatively, if a smaller amount was used, it was necessary to accept that considerable limits were imposed with regard to the maintenance and service life of the lamp by effects such as color drift and an increase in the operating voltage.
The object of the present invention is to provide a metal halide lamp with a ceramic discharge vessel according to the preamble of claim 1 with an improved service life.
This object is achieved by means of the characterizing features of claim 1. Particularly advantageous configurations are given in the dependent claims.
The starting point for the present invention is the discovery that owing to the high temperature in the area of the end face of the lead-through, the rare-earth metal ions from the fill are preferably bonded in the area of a front zone of the lead-through, or at least the surface of that part of the lead-through which is in contact with the discharge volume. This predominantly means the front discharge-side end of the lead-through, since this is where the highest temperature is reached in operation. By contrast, the discharge vessel itself and the sealing means (usually a stopper) are affected to a considerably lesser extent.
Therefore, it may under certain circumstances be sensible to separate the lead-through into a front, particularly halide-resistant part and a rear part which is less at risk. The front part is a cermet component with a ceramic phase and an electrically conductive phase.
Precise investigations have shown that the reaction of the rare-earth metal ions with the ceramic phase of the cermet component results, predominantly in that part of the cermet which is close to the electrodes, in a combination with a chemical composition which, if the ceramic used is aluminum oxide, approximately corresponds to a garnet (RE3Al5O12) or perovskite (REAlO3) or a mixture of the two. The same applies to other ceramics. If this chemically stable composition is reached after a short operating time, it no longer changes during further operation or service life.
If the ceramic phase of the cermet component, whether it be the entire component or a zone on the surface which faces toward the discharge, now contains a considerable proportion (preferably at least 40, in particular more than 80 mol %) of a corresponding combination formed from the ceramic base material and at least one rare-earth metal oxide, the cermet component or its zone which is exposed to the discharge can no longer bind any rare-earth metal from the fill. Therefore, the fill, and consequently the maintenance, of the lamp are stable for a prolonged service life without having to use an excessive quantity of fill. The surface having a garnet or perovskite structure may be located on the front side and, if appropriate, also on the lateral surface of the cermet component.
Specifically, the invention relates to a metal halide lamp with a ceramic discharge vessel in which the discharge vessel has two ends which are closed off by sealing means. One electrically conductive lead-through is guided in a vacuum-tight manner through each of these means, to which lead-throughs an electrode with a shank is attached, which electrode projects into the interior of the discharge vessel. At least a front part of the lead-through, which faces toward the discharge, is designed as a halide-resistant component made from electrically conductive cermet which comprises an electrically conductive (preferably metallic) phase and a ceramic phase, which comprises a ceramic base material. The fill comprises at least one rare-earth metal (i.e. Sc, Y, La and the 14 lanthanides), usually as a halide or as a complex, or alternatively in elemental form. At least on the end face (front side) of the component, at least part of the ceramic phase comprises the combination of the ceramic base material with one or more rare-earth metal oxides.
The cermet component is preferably a pin or a tube. The electrically conductive phase of the cermet is usually a metal, such as molybdenum or tungsten or rhenium or their alloys, or a metal silicide, such as MoSi2.
The most reliable solution, if also the most expensive, is if at least part of the ceramic phase comprises the combination of the ceramic base material with one or more rare-earth metal oxides over the entire length of the component. Preferably, the entire ceramic phase comprises the combination of the ceramic base material and one or more rare-earth metal oxides. The cermet component may form the front part of the lead-through or the entire lead-through.
The ceramic base material is usually polycrystalline aluminum oxide.
In a first embodiment, the rare-earth metal oxides which are used for the cermet component comprise the oxides of one or more or even all of the rare-earth metals which are contained in the fill.
In a second embodiment, the rare-earth metal oxides comprise the oxides of one or more rare-earth metals which are not contained in the fill, in particular Y2O3.
In a third embodiment, a mixture of the first two embodiments is used. In a particularly preferred embodiment, the combination of the ceramic base material with one or more rare-earth metal oxides corresponds to a garnet or perovskite or a mixture of the two. Oxides of La, Nd, Sm, Eu or Gd are preferably used as the perovskite. Oxides of Lu, Yb, Tm and Y are preferably used as the garnet. The remaining rare-earth metal oxides are particularly suitable for both structures and their mixtures.
It is particularly simple and effective for the rare-earth metal oxide used to be predominantly or exclusively an oxide of a rare-earth metal with the smallest possible ionic radius, since it appears that the ions of these rare-earth metals diffuse preferentially into the ceramic phase of the cermet component. In particular, it is sufficient to use a single rare-earth metal oxide with an ionic radius which is less than or equal to the ionic radius of that rare-earth metal ion in the fill which has the smallest ionic radius. An effective ionic radius of up to at most about 0.091 nm is recommended. The scandium ion (Sc3+) is particularly suitable, with a coordination number of 6. This embodiment has the advantage of being independent of the particular choice of fill, so that it can be used for a number of types in common.
It is possible to use this special cermet component for all metal halide lamps with a ceramic discharge vessel, irrespective of whether the sealing is brought about by means of fusible ceramic or by direct sintering.
The special cermet may in principle be produced in a manner known per se by processing a corresponding powder mixture. The fundamental suitability of such materials (in particular yttrium-aluminum garnet) for lamp production is already known, cf. U.S. Pat. No. 5,698,948. However, in that document, the material is used for discharge vessels. By contrast, the requirement for translucency plays no role in the case of lead-throughs.
The sealing means (usually a stopper) advantageously comprises ceramic or cermet (for example suitably doped aluminum oxide), in which case the ceramic base material of the cermet component corresponds to a principal ceramic constituent of the sealing means, in this case, therefore, aluminum oxide. This arrangement has the advantage that the coefficients of thermal expansion of the two parts are similar, so that the cermet component is particularly easy to sinter directly into the stopper.